Abstract

As worldwide electricity demand increases, so does the requirement for effective power conversion combining increased efficiency with minimal harmonic pollution at the lowest financial cost. For medium to high voltage grid-connected applications, multilevel converter topologies enabled the use of lower rated and more efficient self-commutated switches. Due to practical limitations, efficient operation of converters with a low number of levels is restricted to low switching frequencies which in turn becomes a limiting factor for the design of smaller passive filters that are required to limit the associated switching harmonics injected in the AC grid.

This thesis investigates the use of a novel hybrid converter concept aimed at medium-voltage (MV) grid-connected applications. Hybrid converters consist of a main inverter processing the bulk of the power with poor waveform performance and a fast and versatile auxiliary inverter to correct the distortion. In this case, the main converter is a medium-voltage three-level Neutral Point Clamped (NPC) inverter and the auxiliary inverter is a low-voltage and low-current rated Current Source Inverter (CSI), fitted with a series capacitor that is used to minimise the CSI voltage stress. As a result the added installed power by the auxiliary CSI switches can remain at very low levels (theoretically <4%), resulting in a minimal added cost, whilst offering a substantial harmonic improvement to the main VSI. Furthermore the auxiliary converter can be retro-fitted to an existing MV inverter installation to improve the current harmonic quality as required by new grid standards, at a minimal cost.

The performance of the proposed hybrid solution is evaluated through simulation at 3.3 kV MV level under various grid interconnection scenarios. The feasibility of the concept is validated experimentally, scaled to 415V grid voltage level (a more realistic level for a laboratory demonstrator) while operating under more challenging conditions such as switching ripple levels of 50% peak relative to the fundamental peak, showing that the added installed power can be as low as 7% with very high output grid current quality under all grid scenarios considered.